Recently, in order to promote improvements in traffic flow and promote improvements in fuel efficiency through reductions in air resistance, a technology has received attention, which causes a plurality of vehicles to run in a row with a short inter-vehicle distance. In the related art, as a technology of the row running, a method is known in which the respective succeeding vehicles of the row control the inter-vehicle distance between the succeeding vehicle and the preceding vehicle, respectively. In this method, there is a problem in that, when the lead vehicle encounters a disturbance such as the road gradient or wind, an error in inter-vehicle distance is propagated to the rear vehicles. In regard to the problem, a system of a publication of Japanese Unexamined Patent Application Publication No. 10-162282 has been suggested. In this system, each of the succeeding vehicles of the row controls the inter-vehicle distance between itself and the lead vehicle of the row, whereby row running by a plurality of vehicles is realized. According to the method, since the respective succeeding vehicles respond rapidly to the disturbance encountered by the lead vehicle of the row, there is no propagation and the like of an error in inter-vehicle distance, and it is difficult for the behavior of the row to be disturbed.